Hydrogen-bonding interaction between the T1 state of 2-nitrofluorene and alcoholic molecules: Enhancing the oxidative reactivity of the triplet state

Abstract

2-Nitrofluorene (2-NF) is one of the most abundant nitro polycyclic aromatic hydrocarbons (NPAHs), airborne pollutants, and is known for their enhanced toxicity and potential health impacts. This study investigates the reactivity of the T1 state of 2-nitrofluorene, denoted as 2-3NF, toward proton, electron, and hydrogen donors in both polar aprotic and protic solvents, using nanosecond transient absorption and resonance Raman spectroscopies. For the first time, we characterize 2-3NF in various solvents using these techniques. Our results reveal an acid–base equilibrium constant pKa of 1.8 for 2-3NF, indicating much stronger basicity compared to other NPAHs in their T1 state. Resonance Raman and TA spectroscopic analyses confirm the presence of hydrogen-bond interaction between the 2-3NF and alcohol solvent molecules. This interaction significantly enhances the reduction potential of 2-3NF and influences its photoreduction reactivity with substrates such as 1,4-cyclohexadiene and 1-naphthol, where the electron transfer coupled with proton transfer is the primary mechanism. These results provide valuable insight into the role hydrogen bonds play in modulating the reduction reactivity and oxidation potential of NPAHs in their excited states. Furthermore, this study highlights the complex influence of solvent interactions on photochemical processes, emphasizing the importance of incorporating these effects in atmospheric chemistry studies.